A few photomacrographic lenses

[enricosavazzi]

It is already known that some "classic" photomacrographic lenses, including the Zeiss Luminar 63 mm f/4.5, are relatively good (or at least usable) in NUV down to 360-370 nm. I never tested this specific lens in UV because so far I did not have a real need for UV photomacrography, and because I already have better alternatives of similar focal lengths for low-magnification work (Coastalopt 60 mm and UV Rodagon 60 mm).

 

On the other hand, I happen to have a way too large collection of lenses and microscope objectives that I use for photomacrography in the VIS range. Today I took out a few of them and tested them on Micro 4/3 with a stack of adapters that includes a Baader U filter between lens and sensor, and adds 29 mm between lens and camera lens mount. I have many more similar lenses that will have to wait their turn (and may never be tested in the UV unless a good reason arises). I only tested focal lengths around 20-25 mm.

 

First the test subject, a drying branch with a few fully developed snow berries and a group of much smaller abortive berries at the tip of a branch. The abortive berries are 3-3.5 mm long, including the calyx at their tip. The reference lens used in this image is the Coastalopt 60 mm. Light source except when noted below is Bowens 1500Pro with non-coated tube and non-coated dome. Filter is Baader U for NUV images and Baader UVIR cut for VIS image.

 

VIS and NUV image with Coastalopt 60 mm:

 

Light source for framing and focusing was an MTE (www.mte-led.com) with 3W Nichia Single-chip LED. This torch was more than adequate for this purpose. In live view, and in a couple of initial test pictures, the prevalent false color with the tested lenses is yellow, which is the same as with the Coastalopt 60 mm.

 

The first lens is JML 21 mm f/3.5, a known workhorse in photomacrography and a relatively recent discovery (4-5 years) from photomacrography.net. The initial picture was accidentally overexposed. I manually edited it to reduce luminance and restore what was available of its dynamic range. The second picture was instead correctly exposed. I discuss these two images later on.

 

The second lens is Leitz Photar 25 mm f/2. This is the only lens among those tested that has an adjustable aperture. The first image at f/2, the second at f/3.5. This lens is known to perform poorly fully open, so no surprises here. The f/2 aperture is nonetheless useful to precisely focus, because its DOF is minimal.

 

The two following lenses are designed for microfilm/microfiche readers. They have simple optical formulas with simple coatings, and they don't need to filter out UV in their intended use, so it is not surprising that they transmit acceptable amounts of NUV. I don't have the specific data of these lenses, but FL is between 20 and 30 mm and fixed aperture around f/2.8-4.

 

Now some reflections. First, why does changing illumination intensity (and nothing else) obviously change the false color with the JML 21 mm? I think the answer is quite simple. Like all the lenses in this sample, it transmits reasonably well at 380-400 nm, but less well at 360-370 nm. Therefore, with proper exposure, the prevalent false color is the violet of 380-390 nm. This is recorded in the blue and (mostly) red channels. The 390-400 nm false-color blue is sufficiently attenuated by the Baader U to be unnoticeable. Overexposing, on the other hand, saturates the red channel, and records enough of the 360-370 nm "yellow" in the green channel (yellow = red + green in a Bayer sensor). There is actually quite a lot of blue mixed in, which makes the yellow "pale" or slightly "hazel-brown". Recovering the dynamic range cannot restore the blanked-out red channel, which remains at max, and the overblown areas are therefore visible mostly as yellow. It is nonetheless interesting to see one way for false color to appear and disappear in our UV images depending just on proper exposure or overexposure, without actual changes in illumination or reflectance spectrum (other ways for false color to appear and disappear without changes of the spectral distribution of illumination are also possible). I am not saying say that we must never use false color as an indication of UV wavelengths, quite the contrary. Just that there are multiple reasons for false color to appear or disappear in UV images, including spectral distribution and/or selective saturation of one or two sensor color channels.

 

Second, all tested lenses transmit enough NUV to provide useful images. The flash unit was adjusted within a range of about 3 stops to give equivalent exposure, and the changes in magnification are responsible for perhaps 1 stop of the required change. As a whole, the false color is comparable with what we can expect from EL Nikkors and other enlarger lenses. The subject itself does not display "yellows" even with the reference lens, although its color balance is obviously biased toward shorter wavelengths than the tested lenses. All tested lenses are usable. They are designed with fixed apertures (except the Photar) optimized for VIS and for higher magnification than used in this test, so DOF is very small. None of these lenses has a potential as a wideangle (not even in close-up photography). They remain useful only in macrophotography. Flower season is over here, but next year they might become useful. With an added helicoid, magnifications above 4-5x should not be a problem.

[Alex H]

Can you show us how the no-name lenses lok like, or are they displayed on your web site?

[enricosavazzi]

I had in fact forgotten that I have pictures of the lenses on my web site, and that the data of some of these lenses is also there:

 

http://savazzi.net/photography/accidentalphotomacro1.html

 

The first of the two microfilm/microfiche reader lenses I tested above is the JML 26 mm. The other is second and third from the left in the figure immediately above the text "Potential candidates for photomacrographic lenses". I bought the first on eBay, the latter most likely from Surplus Shed.

[Alex H]

Thanks, Enrico. It turns out I have the Surplus Shed lens too.